The present invention relates to an improvement in a marine vessel such as a cantilever jack-up drilling barge or the like for use in underwater operations. More particularly, the present invention relates to an improved offshore drilling rig assembly.
In relatively shallow offshore oil drilling operations of the type conducted from an offshore platform or a jack-up type barge it is conventional for the external drive pipe, or conductor, to be free standing from the ocean floor to a point above the surface of the water and just below the drilling rig. The drive pipe is driven into the ocean floor with a pile driver to a depth of 200 or 300 feet and is cemented to the ocean floor at the mudline.
The drive pipe encloses a concentric arrangement of one or more drill pipe casings and a drill string. The drive pipe provides lateral support for the casings and drill strings from the drilling rig to the well bore and also typically provides support for the pressure control equipment normally referred to as "blow out preventers". Alternatively, the drill pipe casings may be supported at the level of the ocean floor. In either arrangement, the drive pipe provides fluid communication to the drill hole as well as support for the blow out preventers and drill pipe casings.
The drive pipe and drill pipe casings terminate just before the pipe column reaches the drilling platform. The drill string continues through the pressure control equipment affixed to the end of the pipe column.
The action of the ocean waves on the free standing pipe column comprising the drive pipe and drill pipe casings causes the column to sway. Such wave-induced lateral displacement of the pipe interferes with normal drilling and producing operations.
The magnitude of the pipe column's lateral displacement can be reduced if the pipe column is provided with lateral support from the barge. This may be accomplished by connecting to the barge a restricting assembly that hangs beneath the barge and grips the outer pipe of the pipe column. Such lateral support stabilizes the pipe column against the laterally dislocating forces of the ocean waves.
Some special problems must be overcome to attach a restricting assembly to a cantilever type jack-up drilling barge. The cantilever beams which extend outboard over the ocean during drilling and production operations must be retracted when the barge is to be moved to a new location. If the restricting assembly that hangs beneath the drilling platform to stabilize the pipe column is permanently affixed below the level of the cantilever beams, then the restricting assembly would interfere with the retraction of the cantilever beams. Since the deck of the barge under the cantilever beams is used for storage of drill pipe, and since that area must remain free of obstruction, a restricting assembly permanently affixed above the level of the cantilever beams is also not practical.